1. Field of the Invention
The present invention relates to a digital-to-analog converter, a current source and a differential amplifier. More particularly, the present invention relates to a current-driven digital-to-analog converter, as well as a current source and a differential amplifier which are preferably used in the digital-to-analog converter.
2. Description of the Related Art
A typical example of a conventional current-driven digital-to-analog converter (hereinafter also referred to as a D/A converter) is a current cell matrix D/A converter. The current cell matrix D/A converter is well described in Analog Integrated Circuit Design written by David Johns, pp. 477-478, and therefore is not described in detail here.
FIG. 9 shows an example circuit structure of the current cell matrix D/A converter. This D/A converter includes current cells arranged in a matrix as shown at the right of FIG. 9, and each of the current cells (corresponding to one bit in a digital code) has a structure shown at the left of FIG. 9.
The current cell matrix D/A converter converts values of digital codes to amounts of electric current, and is characterized in that variation among output currents is small in relation to variation among the characteristic of metal oxide semiconductor field-effect transistor (MOSFET) devices forming the D/A converter. Therefore, this type of D/A converter is widely recognized as one which performs highly accurate conversion.
However, in the current cell matrix D/A converter such as that described above, the number of current cells required to form the D/A converter exponentially increases as the number of bits in a digital code to be converted by the D/A converter increases. Therefore, there has been a problem in that, when the D/A converter is adapted for use with digital codes having a multi-bit structure, the size of a module thereof becomes large. This problem is particularly serious when the D/A converter is contained in a chip as a semiconductor integrated circuit, since an area for the D/A converter within the chip is limited.
In order to solve the above-described problem, the present invention provides a digital-to-analog converter which can be adapted for use with multi-bit digital codes without significantly increasing the size of a module thereof, as well as a current source and a differential amplifier, which are preferably used in the digital-to-analog converter.
In order to accomplish these objects, a first aspect of the present invention is a current-driven digital-to-analog converter comprising: a constant current source for supplying a current corresponding to the least significant bit in a digital code to be converted into an analog signal; at least one resistor for generating at least one voltage corresponding to at least one bit other than the least significant bit in the digital code; at least one field-effect transistor including at least one control terminal, to which the voltage generated by the resistor is applied, and permitting passage of at least one current corresponding to the bit other than the least significant bit in the digital code; a current source for providing, together with the resistor, the voltage applied to a control terminal of the field-effect transistor, which voltage makes the field-effect transistor operate in a sub-threshold region and also makes the field-effect transistor permit passage of the current corresponding to the bit, to which the field-effect transistor corresponds; and a generator for generating the analog signal based on the current passing through the field-effect transistor and the current supplied by the constant current source.
If there are two or more bits other than the least significant bit (LSB) in the digital code, the resistors are required to generate voltages having mutually different values which respectively correspond to the two or more bits, and the field-effect transistors are required to permit passage of currents of mutually different amounts which also respectively correspond to the two or more bits. Therefore, the required number of the resistors and the field-effect transistors is determined according to the number of the bits other than the LSB, and each of the voltages having mutually different values respectively generated by the different resistors is applied to the control terminal of one of the field-effect transistors.
The field-effect transistors include MOSFETs, high-electron mobility transistors (HEMTs), or the like. The control terminals correspond to gate terminals of the field-effect transistors.
It should be noted that the generator in the present invention can generate an analog signal by, for example, validating only the currents which are permitted to pass through the field-effect transistors corresponding to high-level bits among the bits other than the LSB in the digital code; with respect to the LSB, validating the current supplied by the constant current source only when the LSB is a high-level bit; and then generating the analog signal so that it has a value which is equal to a sum of the amounts of the valid currents. Whether the currents are validated or not is controlled by using switching elements (such as field-effect transistors) which can permit or not permit passage of the respective currents through the respective field-effect transistors. The switching elements are then controlled so as to permit passage of the current only when it is validated. Alternatively, whether the currents are validated or not may be determined by a central processing unit (CPU) on the basis of the digital code.
That is, the present invention utilizes the fact that weights of bits in a digital code differ from each other by a power of two such that a weight of the first bit is 21, a weight of the second bit is 22, and so on, and that the sub-threshold region of the field-effect transistor is, as shown in FIG. 2, a region where the logarithm of a drain current changes linearly with respect to linear change of a gate-source voltage. By setting the voltages applied to the control terminals (gate terminals) of the field-effect transistors so as to make the field-effect transistors operate in the sub-threshold region and also to make the field-effect transistors permit passage of the currents corresponding to the bits, to which the field-effect transistors respectively correspond, the currents, each having one of the amounts corresponding to one of the bits other than the LSB in the digital code, can each be respectively obtained through one of the field-effect transistors and one of the resistors. It should be noted that, since the current corresponding to the LSB in the digital code is supplied by the constant current source, the relevant current is always stable.
Therefore, the digital-to-analog converter of the present invention can be adapted for use with multi-bit digital codes by increasing the number of the field-effect transistors, which contribute to digital-to-analog conversion, so as to correspond to the number of the bits in the digital code, without significantly increasing the size of a module thereof, particularly in comparison with the case of the above-described current cell matrix D/A converter.
As described above, in the digital-to-analog converter according to the first aspect of the present invention, the constant current source supplies the current corresponding to the LSB in the digital code to be converted into an analog signal, the resistors generate the voltages corresponding to the bits other than the LSB in the digital code, and as the generated voltages are applied to the control terminals of the field-effect transistors, the field-effect transistors permit passage of the currents corresponding to the bits other than the LSB in the digital code. Here, the current source provides, through the resistors, the voltages to be applied to the control terminals of the field-effect transistors, which voltages can make the respectively corresponding field-effect transistors operate in the sub-threshold region, and also make the respectively corresponding field-effect transistors permit passage of the currents corresponding to the bits, to which the field-effect transistors respectively correspond. Then, the analog signal is generated based on the currents which pass through the field-effect transistors and the current supplied by the constant current source. Therefore, an increase in the size of the module can be suppressed even when the module is adapted for use with multi-bit digital codes.
The effect of suppressing an increase in the size of the module of the digital-to-analog converter according to the present invention is remarkable when the number of bits in the digital code to be converted is large.
For example, when a digital code having n bits is converted using the above-described current cell matrix D/A converter, the D/A converter needs 2n current cells. On the other hand, the digital-to-analog converter of the present invention needs only n current cells to convert the n-bit digital code. In the case of converting a 10-bit digital code into an analog signal, the area occupied by current cells in the digital-to-analog converter of the present invention is about {fraction (1/100)} of that in the current cell matrix D/A converter.
A second aspect of the present invention is the digital-to-analog converter of the first aspect, further comprising a differential amplifier including a field-effect transistor at an output stage thereof, the field-effect transistor operating in a saturation region and the differential amplifier supplying at least one current for causing the resistor to generate the voltage corresponding to the bit other than the least significant bit in the digital code on the basis of the current supplied by the constant current source.
In the differential amplifier in this aspect, the field-effect transistor provided at the output stage of the differential amplifier operates in a saturation region. That is, the currents for making the resistors generate the voltages corresponding to the bits other than the LSB in the digital code are supplied by the differential amplifier which includes, at the output stage thereof, the field-effect transistor operating in the saturation region, thereby suppressing variance in the currents due to changes in environmental conditions such as temperature, humidity, and the like. As a result, accuracy of the ultimately obtained analog signal can be improved.
As described above, the digital-to-analog converter of the second aspect of the present invention has the same effects as the first aspect of the present invention, and can further improve accuracy of the generated analog signal, since the currents, which make the resistors generate the voltages corresponding to the bits other than the LSB in the digital code on the basis of the current supplied by the constant current source, are supplied by the differential amplifier which includes, at the output stage thereof, the field-effect transistor operating in the saturation region.
In the second aspect of the present invention, the field-effect transistor provided at the output stage of the differential amplifier operates in the saturation region, while the field-effect transistors for permitting passage of the currents corresponding to the bits other than the LSB operate in the sub-threshold region. Therefore, different voltage sources are necessary for these two types of field-effect transistors. However, use of a plurality of voltage sources is not advantageous because it leads to increases in the number of noise sources and in the size of the module.
Therefore, a third aspect of the present invention is the digital-to-analog converter of the second aspect, wherein the field-effect transistor provided at the output stage of the differential amplifier comprises a neuron MOS field-effect transistor.
The neuron MOS field-effect transistor (hereinafter also referred to as xe2x80x9cneuron MOSFETxe2x80x9d) is a functional device invented in 1989 by Dr. Tadashi Shibata. Details of the neuron MOSFET are described in CMOS Analog Circuit Design Technology compiled under the supervision of Dr. Atsushi Iwata, pp. 251-268, and therefore are not described in detail here. As shown in FIG. 6, this type of field-effect transistor includes an input gate terminal and a control gate terminal, and a threshold voltage of the field-effect transistor can be independently controlled according to a potential at the control gate terminal. It should be noted that the threshold voltage here is, as shown in FIG. 8 for example, an extrapolated value of the gate-source voltage, above which a drain current flows.
The threshold voltage in a conventional MOSFET is uniquely determined depending on process conditions. On the other hand, in the neuron MOSFET, the threshold voltage can be independently controlled by controlling the potential at the control gate terminal.
In the present invention, the neuron MOSFET is provided at the output stage of the differential amplifier, and the threshold voltage is adjusted by controlling the voltage to be applied to the control gate terminal of the neuron MOSFET, thereby making the neuron MOSFET operate in the saturation region even when the source voltage is low. This allows one voltage source to be used both as the voltage source for the neuron MOSFET provided at the output stage of the differential amplifier and as the voltage source for the field-effect transistors for supplying the currents corresponding to the bits other than the LSB.
As described above, the digital-to-analog converter of the third aspect of the present invention has the same effects as the first aspect of the present invention, and since it comprises the neuron MOSFET at the output stage of the differential amplifier, the required number of the voltage sources can be reduced to one and an increase in the size can be suppressed.
It is known that a resistance of a resistor decreases as the operating temperature rises, and an amount of current which flows through the resistor increases. However, in the present invention, it is not preferable if the amounts of current that flow through the resistors change depending on the temperature, since accuracy of the generated analog signal is thereby lowered.
In this regard, a fourth aspect of the present invention is any of the digital-to-analog converters of the first to the third aspects, wherein the current source operates so that an amount of current flow decreases as the temperature rises.
That is, in this aspect, if the amounts of current flowing through the resistors change due to a change in temperature, the amount of current supplied by the current source changes so as to compensate for the change, thereby suppressing variance in the voltages generated by the resistors due to the temperature change.
As described above, the digital-to-analog converter of the fourth aspect of the present invention has the same effects as any of the first to the third aspects of the present invention, and since the amount of current flow decreases as the temperature rises, variance in the voltages generated by the resistors due to changes in temperature can be suppressed. As a result, accuracy of the generated analog signal can be improved.
A fifth aspect of the present invention is the digital-to-analog converter of any of the first to the fourth aspects of the present invention, wherein the field-effect transistors which permit passage of the currents corresponding to the bits other than the least significant bit in the digital code comprise neuron MOS field-effect transistors.
Therefore, the digital-to-analog converter of the fifth aspect has the same effects as any of the first to the fourth aspects of the present invention, and since the field-effect transistors thereof, which permit passage of the currents corresponding to the bits other than the LSB in the digital code, comprise the neuron MOS field-effect transistors, drain currents of the neuron MOS field-effect transistors can be independently controlled by respectively controlling the voltages to be applied to control gate terminals of the neuron MOS field-effect transistors. As a result, accuracy of the digital-to-analog conversion can be adjusted after the device has been produced.
It should be noted that a current source of a sixth aspect of the present invention and a differential amplifier of a seventh aspect of the present invention are for use in the digital-to-analog converter of the present invention, and correspond to the current source of the fourth aspect and the differential amplifier of the third aspect, respectively.
Therefore, by applying the current source of the seventh aspect of the present invention to the digital-to-analog converter of the present invention, variance in the voltages generated by the resistors of the digital-to-analog converter due to changes in temperature can be suppressed, thereby improving accuracy of the generated analog signal. Further, by applying the differential amplifier of the seventh aspect of the present invention to the digital-to-analog converter of the present invention, the required number of the voltage sources can be reduced to one and an increase in the size can be suppressed.